We thank Loyola University Chicago, Merck, Co., Inc., and the NIH/National Institute of General Medical Sciences (GM128126) for financial support. We thank Mettler-Toledo Autochem Inc. for their support in the production of this article.

1. Open-air reference spectrum
<ol>
	<li>Open the data acquisition software. Click Instrument. Under the Configure tab, click Collect Background. Click Continue. Set scans to 256 and click OK to collect a background. 
	NOTE: Make sure the probe is in the same position in which data collection will take place. Position changes of the probe may impact spectra.</li>
</ol>
2. Solvent reference spectrum
<ol>
	<li>In the data acquisition software, click File. Click New. Click QuickStart.</li>
	<li>Set Duration to 15 min and Sample Interval to 15 s. Click Create to create experiment. 
	NOTE: At this point, the chemical system must be attached to the in situ IR probe to proceed. The following steps are for the preparation of the chemical system to be studied.</li>
	<li>Under inert atmosphere, add Lewis acid to a flame-dried 25 mL 2-neck round bottom flask charged with a stir bar (Figure 1B). Seal the flask with rubber septa and attach an Ar-filled balloon to the flask. Add desired volume of anhydrous solvent via syringe (minimum 3 mL) (Figure 1C). 
	NOTE: FeCl3 is not soluble in dichloroethane (DCE). GaCl3 is soluble in DCE.</li>
	<li>Remove one septum and attach the flask to the in situ IR probe (Figure 1D). Place the flask in a temperature-controlled bath set to desired temperature (Figure 1E).</li>
	<li>Start the experiment in the data acquisition software by clicking the <img alt="Icon" src="/files/ftp_upload/60745/icon.jpg" /> button to begin collecting data, and stop collecting data after 2 min. 
	NOTE: The name of this file is the solvent reference spectrum that you will use in step 3.1.3.</li>
</ol>
3. Titration software setup
<ol>
	<li>Creating new titration experiment
	<ol>
		<li>In the data acquisition software, click File | New | Quick Start. Set Duration to 8 h and Sample Interval to 15 s. 
		NOTE: The data acquisition has the ability to set experiment duration between 15 min and 2 d and sample interval between 15 s and 1 h.</li>
		<li>Click Create to create experiment. In the data acquisition software, go to Spectra tab and click Add Spectra. Click From File and open appropriate solvent reference spectrum obtained in step 2. Check the box with the time signature. Click OK.</li>
		<li>Start experiment in the data acquisition software by clicking the <img alt="Icon" src="/files/ftp_upload/60745/icon.jpg" /> button to begin collecting data.</li>
	</ol>
	</li>
	<li>Click Solvent Subtraction and select appropriate reference spectrum added in step 3.1.3. Stir for 15 min to reach temperature. Use the in situ IR probe to determine temperature.</li>
</ol>
4. Titration procedure
<ol>
	<li>Add 10 &#181;L of carbonyl analyte via syringe (Figure 1F).</li>
	<li>Observe signal response on the data acquisition (Figure 2). System will shift from equilibrium and change with time.</li>
	<li>When the IR signal stabilizes and remains constant, collect IR spectrum. 
	NOTE: The data acquisition collects spectra at a set frequency. Data in our lab are collected every 15 s. We note the time at which the system reaches equilibrium and use the spectrum collected at the time for analysis.</li>
	<li>Repeat steps 4.1-4.3 until desired amount of analyte is added. 
	NOTE: FeCl3 mixture becomes homogeneous once 1 equiv 1 is added and GaCl3 mixture remains homogeneous regardless of amount of 1 added.</li>
</ol>
5. Analysis of IR spectra
<ol>
	<li>Export data for the data acquisition software.
	<ol>
		<li>Click File | Export | Multi-spectrum file.</li>
		<li>Under Format, check CSV and under Data, check Raw. Click Export to export IR data to spreadsheet or mathematical processing software.</li>
	</ol>
	</li>
	<li>Plot desired region of IR spectrum, as shown in Figure 3A,D.</li>
	<li>Examine the spectrum for transitions and/or isosbestic points.</li>
	<li>Separate spectra by progression, as shown in Figure 3B,C for GaCl3 and Figure 3E,F for FeCl3.</li>
</ol>
6. Component analysis
<ol>
	<li>Identify &#955;max of each species of interest, as shown in Figure 4A for GaCl3 and 1 and Figure 4D for FeCl3 and 1, to generate a table of Absorbance vs. equivalent of analyte added, as shown in Figure 4B for GaCl3 and Figure 4E for FeCl3.</li>
	<li>To account for dilution, multiply the absorbance by the total volume of the solution for each spectrum, as shown in Figure 4B for GaCl3 and Figure 4E for FeCl3.</li>
	<li>Plot product of absorbance*volume as a function of equivalents of analyte, as shown in Figure 4C for GaCl3 and Figure 4F for FeCl3.</li>
</ol>
7. Analysis of consumption of species
<ol>
	<li>For in situ-generated species that can be identified, plot a Beer-Lambert relationship, as shown in Figure 5A.</li>
	<li>For known species, measure the impact of concentration on Absorbance at the desired &#955;max and plot a Beer-Lambert relationship.</li>
	<li>Using the two Beer-Lambert relationships, determine the observed in situ amounts of the species of interest, as shown in Figure 5B. 
	NOTE: CMAX = 2 mmol as defined by the amount of FeCl3 present. CADD is the moles of acetone (1) added. CCOORD is the moles of FeCl3-acetone complex (3). COBS is the moles of unbound 1. CND is the moles of 1 not detected. CMAX &#8211; CCOORD is the moles of 3 that have been consumed.</li>
	<li>Plot CND vs. (CMAX &#8211; CCOORD) to determine if there is a correlation, as shown in Figure 5C. 
	NOTE: The slope of this line will be in moles of species 1 per moles of species 3.</li>
</ol>

<ol>
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This article was produced with support by Mettler-Toledo Autochem Inc., which is the producer of the instrument utilized in this article.

Under anhydrous conditions, Lewis acids can have a range of solubilities. The two examples we have presented are GaCl3 and FeCl3 in DCE. GaCl3 is homogeneous at the onset of the titration, while FeCl3 is largely insoluble. Beginning with the homogeneous solution of GaCl3, we completed a titration from 0-4 equiv 1 in 10 &#956;L increments and extracted the IR spectra (Figure 3A). Examination of the transitions that occur over the course of the titrations shows a formation of a single species in the carbonyl region at 1630 cm-1, which grows from 0-1 equiv 1 (Figure 3B) 26,27. When greater than 1 equiv 1 is added to the solution, no change in the peak at 1630 cm-1 occurs and unbound 1 is observed at 1714 cm-1 (Figure 3C). These results are consistent with the formation of 2. When the same titration is performed with FeCl3 (Figure 3D), a peak at 1636 cm-1 forms from 0-1 equiv 1, which is consistent with 3 (Figure 3E). Importantly, the mixture becomes homogenous once 1 equiv 1 is achieved. When the titration proceeds beyond 1 equiv 1, unbound 1 is observed at 1714 cm-1, 3 decreases in intensity, an isosbestic point resolves at 1648 cm-1, and a new peak at 1663 cm-1 forms.
Using the titration IR data, the equivalents of analyte used can be employed to perform Component Analysis of the solution interactions (Figure 4). To account for dilution, we can employ a normalization with respect to volume of the Beer-Lambert equation (eq. 1):
<img alt="Equation 1" src="/files/ftp_upload/60745/60745equ01.jpg" />
where 1) both absorbance (A) and volume (V) are measurable terms; 2) molar absorptivity (&#949;) and pathlength (l) are constant, allowing 3) number of moles (n) to be examined. The normalized absorbance can easily be computed in a spreadsheet (Figure 4B,D), and then this term can be plotted against equivalents of analyte. In Figure 4C, we can see that the signal for 2 increases linearly with respect to 1 until 1 equiv, at which point the signal for 1 increases linearly and 2 is unchanged. In Figure 4F, we see a similar linear increase in the signal of 3 to 1 equiv 1, followed by the presence of 1 beyond 1 equiv added. However, we also observe a linear decrease in the intensity of 3, and we observe less 1 than we should, assuming similar behavior to GaCl3.
Yet more information is available from the IR data for the titration of FeCl3 with 1. The maximum amount of 3 that can form is defined by the amount of FeCl3 added (CMAX = 2 mmol FeCl3 in the example titration). We know the amount of 1 we add to the flask (CADD), and we can measure the amount of unbound 1 we observe at 1714 cm-1 (COBS) and the amount of 3 we observe at 1636 cm-1 (CCOORD) using Beer-Lambert relationships. Lastly, we know we cannot account for all of the 1 added to the flask as free 1 or 3, indicating that some 1 is not detected (CND). We can combine these terms for 1 in the following mass balance (eq. 2):
<img alt="Equation 2" src="/files/ftp_upload/60745/60745equ02.jpg" />
We can use the titration data to calculate the values of these terms in each IR spectrum generated during the titration (Figure 5B). With these values, we can plot the amount of 1 missing (CND) as a function of the amount 3 consumed (CMAX-CCOORD) to determine if there is a correlation (Figure 5C). This correlation is consistent with 3 equiv 1 consuming 1 equiv 3, which may form a complex similar to 4. We have obtained further support for this number of attached ketones via examination of solution conductivity, which is consistent with one or more of the chlorides being displaced to the outer sphere of Fe(III), and X-ray crystallography of an analogous structure with benzaldehyde12. However, it is likely that there is a mixture of different types of highly-ligated structures that are formed in solution, as is indicated by our non-whole number slopes in our consumption analysis in Figure 5, and the crystal structure we observe may simply be the one complex that precipitates.
In addition to the interactions between two species, this method can be used to probe competitive interactions (Figure 7). By establishing the formation and spectral properties of 3 (Figure 7A) and 5 (Figure 7B), the competition of carbonyls for access to the Lewis acid can be observed. By preforming 3 in solution, we can examine how 6 displaces 1 (Figure 7C). When we probe this system, we see that as we add 6 to 3, not all 6 binds to FeCl3. However, we do observe the consumption of 3 with concomitant presence of 1, as well as the formation of 5.
Using this type of competition experiment, we have been able to simulate the state of FeCl3 as a catalyst in carbonyl-olefin metathesis (Figure 8). We previously demonstrated that at low turnovers, carbonyl-olefin metathesis operates via the primary cycle in Figure 8B28. Substrate 7 interacts with FeCl3 to form complex 9 as the resting state of the cycle. Complex 9 then undergoes turnover-limiting [2+2]-cycloaddition to form oxetane complex 10. Retro-[2+2] yields cycloalkene product 8 and 3, which in turn must have the molecule of 1 displaced by a molecule of 7. However, as the [1] increases, 3 is converted to complex 4. Coordinatively saturated 4 then either sequesters FeCl3 or is catalytically competent, resulting in a parallel cycle via ketone complex 11 and oxetane complex 12.
In conclusion, the utilization of in situ IR to monitor the titration of Lewis acids with carbonyl compounds allows chemists to gain insight into Lewis acid/base solution interactions under synthetically relevant conditions. Not only can this technique be employed to identify discrete structures, but it can be employed to observe the transition of one discrete species into another, as well. Findings from this method have been utilized to propose the mechanism of other metathesis reactions29. We are currently using data gathered via this method to facilitate the reactivity of recalcitrant substrates in carbonyl-olefin metathesis, as well as to develop new forms of metathesis reactions. Lastly, the competitive interactions between substrate carbonyls and product carbonyls likely impact other Lewis acid-catalyzed reactions. We are employing this method to examine these other catalytic regimes.

The utilization of Lewis acids to activate substrates containing carbonyls is ubiquitous in organic synthetic methods1,2,3,4. The study of these interactions has relied on solid state X-ray crystallography, as well as in situ NMR spectroscopy2. Limitations of these techniques manifest from artifacts that arise from crystallization, or the inability to probe paramagnetic Lewis acids via NMR analysis. To overcome these issues, chemists have employed infrared (IR) spectroscopy to determine the exact structure of Lewis pairs. Further, IR has been utilized to determine Lewis acidity4,5,6,7,8,9. The Susz lab studied the solid-state interactions of Lewis acids and carbonyls in the stoichiometric regime. Utilizing IR in conjunction with elemental analysis, the Susz group was able to elucidate the structures of neat, 1:1 mixtures of Lewis pairs. This analysis provided a great deal of insight into structural ramifications of the interactions of simple carbonyl compounds with commonly utilized Lewis acids in the solid state, and of particular interest to our lab: FeCl310,11. We posited that we could add to the existing understanding of the interactions of these ou important Lewis pairs via an in situ method that examines synthetically relevant conditions.
In situ IR enables chemists to perform real-time measurements of functional group conversions in situ. These data supply key insights into reaction rates to support hypotheses about the operating mechanisms of a process and to influence of reaction performance. Real-time observations allow chemists to directly track the interconversion of reaction components over the course of the reaction, and the information gleaned can be employed by the synthetic chemist in the development of new compounds and the optimization of synthetic routes and new chemical processes.
Employing in situ IR spectroscopy as a detection method, we probed the substrates and intermediates that participate in the catalytic cycle of metal-mediated carbonyl-olefin metathesis12. The Fe(III)-catalyzed carbonyl-olefin metathesis process, developed by the Schindler lab, exemplifies a powerful method for the production of C=C bonds from functional groups utilized ubiquitously in the construction of complex molecules13,14,15. Since the original report, this process has inspired a plethora of synthetic developments beyond the utilization of Fe(III)16,17,18,19,20,21,22,23,24,25. Importantly, this reaction requires that the Lewis acid catalyst differentiate between a substrate carbonyl and a product carbonyl for successful reactivity. To observe this competitive interaction under synthetically relevant conditions, we combined titration with the continuous observation provided by in situ IR.
We believe this method is of general importance to chemists studying carbonyl-centered reactions catalyzed by Lewis acids. This detailed demonstration aims to help chemists apply this technique to their system of study.

<table><tbody><tr><td>Acetone</td><td>BDH</td><td>BDH1101-19L</td><td>Dried over potassium carbonate</td></tr><tr><td>Balloon</td><td>VWR</td><td>470003-408</td><td>Round Balloons, Assorted Colors, 9&quot; dia.</td></tr><tr><td>Detector LN2 RiR15</td><td>Mettler Toledo</td><td>14474603</td><td></td></tr><tr><td>1,2 Dichloroethane</td><td>Beantown</td><td>223375-2.5L</td><td>Dried over 3&amp;Aring; molecular sieves</td></tr><tr><td>Gallium (III) Chloride</td><td>Beantown</td><td>127270-100G</td><td>Anhydrous &amp;ge;99.999% (trace metals basis)</td></tr><tr><td>25 &amp;micro;L glass syringe</td><td>Hamilton</td><td>80285</td><td></td></tr><tr><td>Inert Argon Gas</td><td>Airgas</td><td></td><td>Ultra High Purity</td></tr><tr><td>Iron (III) Chloride</td><td>Sigma Aldrich</td><td>157740-100G</td><td>Reagent Grade, 97%</td></tr><tr><td>100-mL Jacketed Beaker</td><td>AceGlass</td><td>5340-03</td><td></td></tr><tr><td>3&amp;Aring; Molecular Sieves</td><td>Alfa Aesar</td><td>L05335</td><td></td></tr><tr><td>25-mL 2 neck flask</td><td>CTechGlass</td><td>FL-0143-003</td><td></td></tr><tr><td>18G Needle</td><td>BD Biosciences</td><td>305196</td><td>Needles with Regular Bevel, 38.1 mm (11/2&quot;)</td></tr><tr><td>Potassium Carbonate</td><td>Sigma Aldrich</td><td>60109-1KG-F</td><td>Anhydrous</td></tr><tr><td>Prism 8</td><td>GraphPad</td><td></td><td>Mathematical Processing Software</td></tr><tr><td>Probe DST 6.35 x 1.5m X 203 DiComp</td><td>Mettler Toledo</td><td>14474510</td><td>&lt;em&gt;in situ&lt;/em&gt; IR probe</td></tr><tr><td>Rice Stir Bar</td><td>Dynalon</td><td>303495</td><td>Diameter: 3 mm (1/8&quot;), Length: 10 mm (3/8&quot;)</td></tr><tr><td>14/20 Rubbber Septa</td><td>VWR</td><td>89097-554</td><td></td></tr><tr><td>5-mL Syringe</td><td>AIR-TITE</td><td>53548-005</td><td>HSW Norm-Ject Sterile Luer-Slip Syringes, Air-Tite</td></tr><tr><td>10-mL Syringe</td><td>AIR-TITE</td><td>53548-006</td><td>HSW Norm-Ject Sterile Luer-Slip Syringes, Air-Tite</td></tr><tr><td>System ReactIR 15</td><td>Mettler Toledo</td><td>1400003</td><td>&lt;em&gt;in situ&lt;/em&gt; IR system</td></tr><tr><td>Thermostatic Bath</td><td>Haake</td><td></td><td>Haake A82</td></tr></tbody></table>

characterizing lewis pairs using titration coupled with in situ infrared spectroscopy

Lewis acid-activation of carbonyl-containing substrates is a fundamental basis for facilitating transformations in organic chemistry. Historically, characterization of these interactions has been limited to models equivalent to stoichiometric reactions. Here, we report a method utilizing in situ infrared spectroscopy to probe the solution interactions between Lewis acids and carbonyls under synthetically relevant conditions. Using this method, we were able to identify 1:1 complexation between GaCl3 and acetone and a highly ligated complex for FeCl3 and acetone. The impact of this technique on mechanistic understanding is illustrated by application to the mechanism of Lewis acid-mediated carbonyl-olefin metathesis in which we were able to observe competitive binding interactions between substrate carbonyl and product carbonyl with the catalyst.

In this study, in situ IR-monitored titration was used to observe the interactions of 1 and GaCl3 as well as 1 and FeCl3 (Figure 6)12. Using this collection of protocols, we were able to determine that GaCl3 and 1 form 1:1 complex 2 in solution. Alternatively, when FeCl3 and 1 are combined, more complex behavior is observed. Figure 6 displays the equilibria we were examining. Figure 1 displays the physical setup of the titration of FeCl3 with 1. Figure 2 displays the raw feed of data obtained by the in situ IR using the data acquisition software for the titration of FeCl3 with 1. Figure 3 displays the process of extracting the transitions that result from this titration method applied to GaCl3 and FeCl3. Figure 4 displays the extraction of &#955;max data of the titration of GaCl3 with 1 and the titration of FeCl3 with 1. Figure 5 displays the extraction of complex coordination behavior from the titration of FeCl3 with 1. Figure 7 displays an application of these protocols for the examination of competitive access to a Lewis acid. Figure 8 shows the application of these protocols to revising the mechanism of metal catalyzed carbonyl-olefin metathesis.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/60745/60745fig01.jpg" /> 
Figure 1: Visual guide to system setup. Necessary components for performing the titration (A). Assembled components prior to attachment to the in situ IR (B). Flask with Ar and ready for solvent addition (C). Flask attached to the in situ IR with solvent (D). Flask under temperature control (E). Ready for addition of analyte (F). <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/60745/60745fig02.jpg" /> 
Figure 2: Analyte signal response in the data acquisition interface at 1636 cm-1 for titration of 2 mmol FeCl3 in 12 mL of DCE with 1. The spectrum is collected when the system is at equilibrium, after analyte addition. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/60745/60745fig03.jpg" /> 
Figure 3: Analysis of IR spectra. Spectra collected for the titrations GaCl3 with 0-4 equiv 1 (A) and FeCl3 with 0-4 equiv 1 (D). Breakdown of titration of GaCl3 with 0-1 equiv 1 showing the formation of 2 (B) and with 1-4 equiv 1 showing the presence of 1 (C). Breakdown of titration of FeCl3 with 0-1 equiv 1 showing the formation of 3 (E) and with 1-4 equiv 1 showing the presence of 1, the consumption of 3, and the formation of a new species (F). Reprinted (adapted) with permission from Hanson, C. S., et al12. Copyright 2019 American Chemical Society. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/60745/60745fig04.jpg" /> 
Figure 4: Extraction of &#955;max data from IR for Component Analysis. Spectra collected for the titrations GaCl3 with 0-4 equiv 1 with the &#955;max for 1 and 2 indicated (A) and FeCl3 with 0-4 equiv 1 with the &#955;max for 1 and 3 indicated (D). Table showing representative data normalized to account for dilution for GaCl3 (B) and for FeCl3 (E). Data from (B) plotted for component analysis of titration of GaCl3 with 1 (C) and for component analysis of titration of FeCl3 with 1 (F). Reprinted (adapted) with permission from Hanson, C. S., et al12. Copyright 2019 American Chemical Society. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/60745/60745fig05.jpg" /> 
Figure 5: Consumption analysis of titration of FeCl3 with 1. Segment of IR data used to generate a Beer-Lambert relationship for [3] and the segment of IR data used to determine the consumption of 3 (A). Moles of each 1-containing species measured from IR (B). Plot of moles of 1 not detected vs. moles of 3 consumed (C). Reprinted (adapted) with permission from Hanson, C. S., et al12. Copyright 2019 American Chemical Society. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" class="xfigimg" src="/files/ftp_upload/60745/60745fig06.jpg" /> 
Figure 6: Lewis acid/base equilibria probed in this study. Titrations of GaCl3 with 1 to form 2 and FeCl3 with 1 to form 3 and 4 are reported. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig06large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" class="xfigimg" src="/files/ftp_upload/60745/60745fig07.jpg" /> 
Figure 7: Competitive binding experiment. Carbonyl region of IR spectrum of 3 (A) and of IR spectrum of 5 (B). Equilibrium probed in titration of 3 with 6 (C). IR data of titration of 3 with 1 equiv 6 (D). Reprinted (adapted) with permission from Hanson, C. S., et al12. Copyright 2019 American Chemical Society. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig07large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 8" class="xfigimg" src="/files/ftp_upload/60745/60745fig08.jpg" /> 
Figure 8: Application of in situIR data in mechanistic proposal. Carbonyl-olefin metathesis reaction of 7 (A). The revised mechanistic proposal of carbonyl-olefin metathesis facilitated by titration coupled with in situ IR spectroscopy (B). Reprinted (adapted) with permission from Hanson, C. S., et al12. Copyright 2019 American Chemical Society. <a href="https://jove.unicartagenaproxy.elogim.com/files/ftp_upload/60745/60745fig08large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Here, we present a method for the observation of solution interactions between Lewis acids and bases by employing in situ infrared spectroscopy as a detector for titration under synthetically relevant conditions. By examining solution interactions, this method represents a complement to X-ray crystallography, and provides an alternative to NMR spectroscopy.

Lewis acid-activation of carbonyl-containing substrates is a fundamental basis for facilitating transformations in organic chemistry. Historically, ...

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Unit 1

Acids and Bases

Acid–Base Titration

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9.0K Views.  Loyola University Chicago. Here, we present a method for the observation of solution interactions between Lewis acids and bases by employing in situ infrared spectroscopy as a detector for titration under synthetically relevant conditions. By examining solution interactions, this method represents a complement to X-ray crystallography, and provides an alternative to NMR spectroscopy.

Video: Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

Diffuse Reflectance Infrared Spectroscopic Identification of Dispersant/Particle Bonding Mechanisms in Functional Inks

In additive manufacturing, or 3D printing, material is deposited drop by drop, to create micron to macroscale layers. A typical inkjet ink is a colloidal dispersion containing approximately ten components including solvent, the nano to micron scale particles which will comprise the printed layer, polymeric dispersants to stabilize the particles, and polymers to tune layer strength, surface tension and viscosity. To rationally and efficiently formulate such an ink, it is crucial to know how the components interact. Specifically, which polymers bond to the particle surfaces and how are they attached? Answering this question requires an experimental procedure that discriminates between polymer adsorbed on the particles and free polymer. Further, the method must provide details about how the functional groups of the polymer interact with the particle. In this protocol, we show how to employ centrifugation to separate particles with adsorbed polymer from the rest of the ink, prepare the separated samples for spectroscopic measurement, and use Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) for accurate determination of dispersant/particle bonding mechanisms. A significant advantage of this methodology is that it provides high level mechanistic detail using only simple, commonly available laboratory equipment. This makes crucial data available to almost any formulation laboratory. The method is most useful for inks composed of metal, ceramic, and metal oxide particles in the range of 100 nm or greater. Because of the density and particle size of these inks, they are readily separable with centrifugation. Further, the spectroscopic signatures of such particles are easy to distinguish from absorbed polymer. The primary limitation of this technique is that the spectroscopy is performed ex-situ on the separated and dried particles as opposed to the particles in dispersion. However, results from attenuated total reflectance spectra of the wet separated particles provide evidence for the validity of the DRIFTS measurement.

Formulation of stable, functional inks is critical to expanding the applications of additive manufacturing. In turn, knowledge of the mechanisms of dispersant/particle bonding is required for effective ink formulation. Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) is presented as a simple, inexpensive way to gain insight into these mechanisms.

In additive manufacturing, or 3D printing, material is deposited drop by drop, to create micron to macroscale layers.  A typical inkjet ink is a colloidal ...

Chemical Analysis of Water-accommodated Fractions of Crude Oil Spills Using TIMS-FT-ICR MS

Multiple chemical processes control how crude oil is incorporated into seawater and also the chemical reactions that occur overtime. Studying this system requires the careful preparation of the sample in order to accurately replicate the natural formation of the water-accommodated fraction that occurs in nature. Low-energy water-accommodated fractions (LEWAF) are carefully prepared by mixing crude oil and water at a set ratio. Aspirator bottles are then irradiated, and at set time points, the water is sampled and extracted using standard techniques. A second challenge is the representative characterization of the sample, which must take into consideration the chemical changes that occur over time. A targeted analysis of the aromatic fraction of the LEWAF can be performed using an atmospheric-pressure laser ionization source coupled to a custom-built trapped ion mobility spectrometry&#8211;Fourier transform&#8211;ion cyclotron resonance mass spectrometer (TIMS-FT-ICR MS). The TIMS-FT-ICR MS analysis provides high-resolution ion mobility and ultrahigh-resolution MS analysis, which further allow the identification of isomeric components by their collision cross-sections (CCS) and chemical formula. Results show that as the oil-water mixture is exposed to light, there is significant photo-solubilization of the surface oil into the water. Over time, the chemical transformation of the solubilized molecules takes place, with a decrease in the number of identifications of nitrogen- and sulfur-bearing species in favor of those with a greater oxygen content than were typically observed in the base oil.

The low-energy water-accommodated fraction (LEWAF) of crude oil is a challenging system to analyze, because over time, this complex mixture undergoes chemical transformations. This protocol illustrates methods for the preparation of the LEWAF sample and for performing photo-irradiation and chemical analysis by trapped ion mobility spectrometry&#8211;FT-ICR MS.

Multiple chemical processes control how crude oil is incorporated into seawater and also the chemical reactions that occur overtime. Studying this system ...

Environment

Raman and IR Spectroelectrochemical Methods as Tools to Analyze Conjugated Organic Compounds

In the presented work, two spectroelectrochemical techniques are discussed as tools for the analysis of the structural changes occurring in the molecule on the vibrational level of energy. Raman and IR spectroelectrochemistry can be used for advanced characterization of the structural changes in the organic electroactive compounds. Here, the step-by-step analysis by means of Raman and IR spectroelectrochemistry is shown. Raman and IR spectroelectrochemical techniques provide complementary information about structural changes occurring during an electrochemical process, i.e. allows for the investigation of redox processes and their products. The examples of IR and Raman spectroelectrochemical analysis are presented, in which the products of the redox reactions, both in solution and solid state, are identified.

A protocol of step-by-step Raman and IR spectroelectrochemical analysis is presented.

In the presented work, two spectroelectrochemical techniques are discussed as tools for the analysis of the structural changes occurring in the molecule ...

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids with adsorbates. Vibrational spectra provide information on the chemical nature of adsorbed species and their structure. Thus, they are very useful for obtaining molecular level understanding of surface species. The IR spectrum of the sample itself gives some direct information about the material. General conclusions can be drawn concerning hydroxyl groups, some stable surface species and impurities. However, the spectrum of the sample is &#34;blind&#34; with respect to the presence of coordinatively unsaturated ions and gives rather poor information about the acidity of surface hydroxyls, species decisive for the adsorption and catalytic properties of the materials. Furthermore, no discrimination between bulk and surface species can be made. These problems are solved by the use of probe molecules, substances that interact specifically with the surface; the alteration of some spectral features of these molecules as a result of adsorption provides valuable information about the nature, properties, location, concentration, etc., of the surface sites.
The experimental protocol for in-situ IR studies of gas/sample interaction includes preparation of a sample pellet, activation of the material, initial spectral characterization through the analysis of the background spectra, characterization by probe molecules, and study of the interaction with a particular set of gas mixtures. In this paper we investigate a zirconium terephthalate metal organic framework, Zr6O4(OH)4(BDC)6 (BDC = benzene-1,4-dicarboxylate), namely UiO-66 (UiO refers to University of Oslo). The acid sites of the UiO-66 sample are determined by using CO and CD3CN as molecular probes. Furthermore, we have demonstrated that CO2 is adsorbed on basic sites exposed on dehydroxylated UiO-66. Introduction of water to the system produces hydroxyl groups acting as additional CO2 adsorption sites. As a result, CO2 adsorption capacity of the sample is strongly enhanced.

In situ FTIR Spectroscopy as a Tool for Investigation of Gas/Solid Interaction: Water-Enhanced CO2 Adsorption in UiO-66 Metal-Organic Framework

The use of FTIR spectroscopy for investigation of the surface properties of polycrystalline solids is described. Preparation of sample pellets, activation procedures, characterization with probe molecules and model studies of CO2 adsorption are discussed.

In situ infrared spectroscopy is an inexpensive, highly sensitive, and selective valuable tool to investigate the interaction of polycrystalline solids ...

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional &#960;-conjugate Systems

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near infrared (near-IR) transitions, because it can overcome the low sensitivity of spontaneous Raman spectrometers in the near-IR region. Here, we describe technical details of a femtosecond time-resolved near-IR multiplex stimulated Raman spectrometer that we have recently developed. A description of signal generation and optimization, measurement, data acquisition, and calibration and correction of recorded data is provided as well. We present an application of our spectrometer to analyze the excited-state dynamics of &#946;-carotene in toluene solution. A C=C stretch band of &#946;-carotene in the second lowest excited singlet (S2) state and the lowest excited singlet (S1) state is clearly observed in the recorded time-resolved stimulated Raman spectra. The femtosecond time-resolved near-IR stimulated Raman spectrometer is applicable to the structural dynamics of &#960;-conjugate systems from simple molecules to complex materials.

Details of signal generation and optimization, measurement, data acquisition, and data handling for a femtosecond time-resolved near-IR stimulated Raman spectrometer are described. A near infrared stimulated Raman study on the excited-state dynamics of &#946;-carotene in toluene is shown as a representative application.

Femtosecond time-resolved stimulated Raman spectroscopy is a promising method of observing the structural dynamics of short-lived transients with near ...

Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active metal-oxide surfaces in contact with light-absorbing dyes is an ongoing thorny and elusive challenge in physical chemistry. With this in mind, a molecular-dynamics (MD) simulation were performed by using optimized empirical potentials for a well-representative and prototypical dye-sensitized solar cell (DSC) solvated by a widely-studied room temperature ionic liquid (RTIL), in the guise of a [bmim]+[NTf2]- RTIL solvating an N719-sensitizing dye adsorbed onto 101 anatase-titania. In doing so, important insights were gleaned into how using a RTIL as the electrolytic hole acceptor modulates the dynamical and vibrational properties of a N719 dye, estimating the spectra for the DSC photo-active interface via Fourier transformation of mass-weighted velocity autocorrelation functions from MD. The acquired vibrational spectra were compared with the experiment spectra and those sampled from ab initio molecular dynamics (AIMD); in particular, various empirical-potential spectra generated from MD provide insight into how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction. In any event, careful fitting of empirical force-field models has been shown to be an effective tool in handling DSC vibrational properties, when validated by AIMD and an experiment.

A dye-sensitized solar cell was solvated by RTILs; using optimized empirical potentials, a molecular dynamics simulation was applied to compute vibrational properties. The obtained vibrational spectra were compared with experiment and ab initio molecular dynamics; various empirical potential spectra show how partial-charge charge parameterization of the ionic liquid affects vibrational spectra prediction.

The accurate molecular simulation prediction of vibrational spectra, and other structural, energetic and spectral characteristics, of photo-active ...

Exploring Multiphase Polymeric Systems by Nanoscale Infrared Spectroscopy

Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition is commonly assessed using infrared spectroscopy, which provides an average fingerprint of the various materials contained in the volume probed. However, this approach does not offer any details on the arrangement of the phases in the material. Interfacial regions between two polymeric phases, often in the nanoscale range, are also challenging to access. Photothermal nanoscale infrared spectroscopy monitors the local response of materials excited by infrared light with the sensitive probe of an atomic force microscope (AFM). While the technique is suitable for interrogating small features, such as individual proteins on pristine gold surfaces, the characterization of three-dimensional multicomponent materials is more elusive. This is due to a relatively large volume of material undergoing photothermal expansion, defined by the laser focalization onto the sample and by the thermal properties of the polymeric constituents, compared to the nanoscale region probed by the AFM tip. Using a polystyrene (PS) bead and a polyvinyl alcohol (PVA) film, we evaluate the spatial footprint of photothermal nanoscale infrared spectroscopy for surface analysis as a function of the position of PS in the PVA film. The effect of the feature position on the nanoscale infrared images is investigated, and spectra are acquired. Some perspectives on the future advances in the field of photothermal nanoscale infrared spectroscopy are provided, considering the characterization of complex systems with embedded polymeric structures.

Author Spotlight: Advances in Nanoscale Infrared Spectroscopy to Explore Multiphase Polymeric Systems  

This protocol describes the application of atomic force microscopy and nanoscale infrared spectroscopy to evaluate the performance of photothermal nanoscale infrared spectroscopy in the characterization of three-dimensional multi-polymeric samples.

Multiphase polymeric systems encompass local domains with dimensions that can vary from a few tens of nanometers to several micrometers. Their composition ...

Engineering

Infrared Spectroscopy

Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA
This experiment will demonstrate the use of infrared (IR) spectroscopy (also known as vibrational spectroscopy) to elucidate the identity of an unknown compound by identifying the functional group(s) present. IR spectra will be obtained on an IR spectrometer using the attenuated total reflection (ATR) sampling technique with a neat sample of the unknown.

Infrared Spectroscopy: Characterization of Functional Groups

Source: Vy M. Dong and Zhiwei Chen, Department of Chemistry, University of California, Irvine, CA
This experiment will demonstrate the use of infrared ...

JoVE Science Education

Organic Chemistry II

Infrared (IR) Spectroscopy: Overview

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
Different compounds display unique properties due to their functional groups, which allows IR spectroscopy to be used to determine the present functional groups. Vibrational IR spectroscopy is performed in the 2.5 to 25 &#181;m wavelength range. A&#160; molecule is considered IR active if it exhibits a change in dipole moment during vibration when absorbing IR radiation. For a molecule to absorb IR radiation effectively, its vibrations must fluctuate its dipole moments, allowing it to interact with the electromagnetic field of the IR light.
Since different functional groups absorb IR radiation at varying frequencies, the IR spectrum is similar to the &#34;fingerprint&#34;&#160;for each molecule. IR spectroscopy is primarily employed in qualitative analysis to identify the functional groups in organic and inorganic compounds by comparing their vibrational frequencies with known compounds. Additionally, It can determine the concentration of a substance by applying the Beer-Lambert law, where absorbance is proportional to concentration.

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation ...

Characterizing Lewis Pairs Using Titration Coupled with In Situ Infrared Spectroscopy

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